renderer/native: Don't clear GPU data with pending page flips

An incorrect assumption that after mode set there would be no pending
page flips was made. This meant that if there was a mode set, followed
by a page flip, if that page flip was for a CRTC on a now unused GPU,
we'd crash due to the renderer GPU data having already been freed. This
commit avoids that by keeping it alive as long as the page flips are
still in the air. It fixes crashes with backtraces such as

  0) meta_render_device_get_egl_display (render_device=0x0)
     at ../src/backends/native/meta-render-device.c:320
  1) secondary_gpu_state_free (secondary_gpu_state=0x1c8cc30)
     at ../src/backends/native/meta-onscreen-native.c:560
  2) meta_onscreen_native_dispose (object=0x1cb65e0)
     at ../src/backends/native/meta-onscreen-native.c:2168
  3) g_object_unref (_object=<optimized out>)
     at ../gobject/gobject.c:3540
  4) g_object_unref (_object=0x1cb65e0)
     at ../gobject/gobject.c:3470
  5) clutter_stage_view_finalize (object=0x1cbb450)
     at ../clutter/clutter/clutter-stage-view.c:1412
  6) g_object_unref (_object=<optimized out>)
     at ../gobject/gobject.c:3578
  7) g_object_unref (_object=0x1cbb450)
     at ../gobject/gobject.c:3470
  8) meta_kms_page_flip_closure_free (closure=0x1d47e60)
     at ../src/backends/native/meta-kms-page-flip.c:76
  9) g_list_foreach (list=<optimized out>, func=0x7fb3ada67111 <meta_kms_page_flip_closure_free>, user_data=0x0)
     at ../glib/glist.c:1090
 10) g_list_free_full (list=0x1cb4d20 = {...}, free_func=<optimized out>)
     at ../glib/glist.c:244
 11) meta_kms_page_flip_data_unref (page_flip_data=0x1c65510)
     at ../src/backends/native/meta-kms-page-flip.c:109
 12) meta_kms_callback_data_free (callback_data=0x227ebf0)
     at ../src/backends/native/meta-kms.c:372
 13) flush_callbacks (kms=0x18e2630)
     at ../src/backends/native/meta-kms.c:391
 14) callback_idle (user_data=0x18e2630)
     at ../src/backends/native/meta-kms.c

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2147>
(cherry picked from commit 287908e0)